Air filter and production method therefor

ABSTRACT

An air filter comprising a laminate of a non-woven fabric made up of polyolefin fibers or polyester fibers or a composite of the non-woven fabrics and a PTFE porous film, wherein the amount of the total gases generated from the air filter is 20 μg/g by the analysis of gas chromatography. The amount of the gases generated from the air filter is greatly reduced by heat-treating the non-woven fabric at a temperature of from 80 to 130° C. for 5 minutes to 20 minutes before or after laminating with the PTFE porous film.

FIELD OF THE INVENTION

[0001] The present invention relates to an air filter having lowdegassing (gas generating) property using a polytetrafluoroethylene(PTFE) porous film and to a production method thereof, and the airfilter of the present invention has an excellent performance having noproblem of causing a degassing property (i.e., the generation of gases).

BACKGROUND OF THE INVENTION

[0002] Hitherto, as a material for air filters for clean rooms, a filtermaterial prepared by adding a binder to glass fibers and subjecting theresulting glass fibers to processing by a paper manufacturing machine isfrequently used. However, in such materials, there are various problems.For example, the existence of small fibers attached to the filtermaterial, the increase of a pressure loss in the case of increasing theamount of a binder for preventing the self dust generation at working orbending the filter material (JP-A-63-16019) (the term “JP-A” as usedherein means an unexamined published Japanese patent application), etc.Furthermore, in the filter material, there is also a problem that whenthe filter material comes in contact with a certain kind of chemicalssuch as hydrofluoric acid, the glass and binder are deteriorated tocause the generation of dusts.

[0003] For solving these problems, recently, a PTFE porous film is usedin various fields as the filter material for filter. The PTFE film isuseful as the filter materials for filters used in the fields requiringsevere clean environment, such as a semiconductor production, etc., asdescribed in JP-A-59-152825, 3-221541, 5-202217, and 7-196831, and PCTWO 94/16802).

[0004] Because such a useful PTFE porous film itself has a weakstrength, a non-woven fabric or a woven fabric of polyethylene,polyester, etc., is laminated as a reinforcing material to improve thestrength and the folding workability thereof.

[0005] However, there is a problem that a gas such as2,6-di-tert-butyl-p-cresol (BHT), dibutyl phthalate (DBP), etc., whichis said to give bad influences on the products in the field of thesemiconductor production requiring a severe clean environment, is leakedfrom the non-woven fabric which is laminated as the reinforcingmaterial, though the PTFE is a clean material.

SUMMARY OF THE INVENTION

[0006] The present invention has been made for solving theabove-described problems.

[0007] That is, according to a first aspect of the present invention,there is provided an air filter comprising a laminate of a non-wovenfabric made up of fibers containing at least one selected frompolyolefins and polyesters as the main component or a composite composedof the non-woven fabrics and a polytetrafluoroethylene porous film,wherein the total amount of gases generated from the air filter is notmore than 20 μg/g as a result of quantitative analysis by gaschromatography.

[0008] Also, in the air filter of the present invention, it is preferredthat the non-woven fabric is heat-treated.

[0009] According to a second aspect of the present invention, there isprovided a production method of an air filter, which comprisesheat-treating a non-woven fabric made up of fibers containing at leastone kind selected from polyolefins and polyesters as the main componentor a composite of the non-woven fabrics at a temperature of from 80° C.to 130° C. for from 5 minutes to 20 minutes and thereafter laminatingthe non-woven fabric or the composite thereof with apolytetrafluoroethylene porous film.

[0010] Furthermore, according to a third aspect of the presentinvention, there is provided a production method of an air filter, whichcomprises laminating a non-woven fabric made up of fibers containing atleast one kind selected from polyolefins and polyesters as the maincomponent or a composite of the non-woven fabrics and apolytetrafluoroethylene porous film, and thereafter heat-treating theresulting laminate at a temperature of from 80° C. to 130° C. for from 5minutes to 20 minutes.

DETAILED DESCRIPTION OF THE INVENTION

[0011] Then, the present invention described in detail.

[0012] In the air filter of the present invention, there is noparticular restriction on the non-woven fabric used as the reinforcingmaterial if the non-woven fabric is made up of fibers containing atleast one kind of polyolefins and polyesters as the main components or anon-woven fabric made up of the composite of these non-woven fabricsdescribed above and commercially available products can be used.

[0013] The polyolefins used in the present invention includepolyethylene (PE), polypropylene (PP), etc., and the polyesters used inthe present invention include polyethylene terephthalate (PET), etc.

[0014] The composite non-woven fabric used in the present invention is,for example, composite fibers of a core/sheath structure and includes acomposite non-woven fabric wherein the core component is made up ofsynthetic fibers having a relatively higher melting point than that ofthe sheath component, a double layer non-woven fabric made up of alow-melting material and a high-melting material, a composite non-wovenfabric prepared by fusing a powder of the low-melting material onto thesurface of a non-woven fabric of the high-melting material. Ascommercially available composite non-woven fabrics can be used in thepresent invention and they include, for example, ELEVES T0153WDO (tradename, manufactured by UNITIKA LTD., a composite non-woven fabric of acore/sheath structure of PE and PET), Syntex R-250 (trade name,manufactured by MITSUI PETROCHEMICAL INDUSTRIES, LTD., a compositenon-woven fabric of PETs), and TAPYRUS SN32ESB-SHP10 (trade name,manufactured by Tonen Tapyrus K.K., a composite non-woven fabricprepared by fusing the powder of PE onto the non-woven fabric of PET).

[0015] In the air filter of the present invention, the PTFE porous filmcan be prepared, for example, by the following method.

[0016] First, a fine powder of PTFE is mixed with a liquid lubricant,and the resulting mixture is molded into a sheet form in an unsinteredstate by at least one method of an extrusion method and a rolling methodto obtain a sheet-form molding.

[0017] There is no particular restriction on the above-described finepowder of PTFE and commercially available products can be used. Examplesof the commercially available products include Polyflon F-104 (tradename, manufactured by DAIKIN INDUSTRIES, LTD.), Fluon CD-123 (tradename, manufactured by Asahi•ICI Fluoropolymers K.K.), and Teflon 6J(trade name, manufactured by DU PONT-MITSUI FLUOROCHEMICALS CO., LTD.).

[0018] Also, any liquid lubricant can be used in the present invention,as long as the lubricant can wet the surface of the fine powder of PTFEand can be removed by a method of evaporation, extraction, etc. afterobtaining the above-described sheet-form molding. Examples of the liquidlubricant include hydrocarbon oils such as liquid paraffin, a naphtha, awhite oil, toluene, xylene, etc.; alcohols, ketones, esters, andmixtures of 2 or more kinds of them.

[0019] The addition amount of the liquid lubricant to the fine powder ofPTFE can be selected appropriately depending on the kinds of the finepowder of PTFE and the liquid lubricant and the molding method ofobtaining the sheet-form molding, but the amount of the liquid lubricantis generally from 5 to 50 parts by weight per 100 parts by weight of thefine powder of PTFE.

[0020] As examples of the method of molding a mixture of the fine powderof PTFE and the liquid lubricant into a sheet form by at least onemethod of an extrusion method and a rolling method, there are a methodwhich comprises extruding a mixture of the fine powder of PTFE and theliquid lubricant into a rod form, and rolling the rod into a sheet formby a pair of rolls and a method which comprises extruding the mixtureinto a plate form to form a sheet thereof. The thickness of thethus-obtained sheet form molding is usually from 0.05 to 0.5 mm.

[0021] Then, the liquid lubricant contained in the sheet-form molding isremoved. The removal of the liquid lubricant can be carried out by aheating method, an extrusion method or a combination of these methods.

[0022] Finally, the sheet-form molding from which the liquid lubricantwas removed is stretched to at least one axial direction to make themolding porous. There is no restriction on the stretching temperatureand the stretching ratio (drawing ratio).

[0023] The production method of an air filter according to the presentinvention has the characteristic in that a heat treatment is carried outin laminating the non-woven fabric and the PTFE porous film describedabove to produce a filter. The heat treatment can be effected eitherbefore or after laminating the non-woven fabric and the PTFE porousfilm.

[0024] The method of the heat treatment is not limited, as long as themethod can heat-treating the non-woven fabric or the PTFE porous filmair filter at a temperature of from 80° C. to 130° C. for from 5 minutesto 20 minutes. But, a method of passing the non-woven fabric or the PTFEporous film air filter through the inside of a heating furnace whilefixing both ends of the non-woven fabric or the air filter and a methodof moving the non-woven fabric or the PTFE porous film air filter alonga heated roll are preferred because a heat stress is hard to occur.

[0025] As a method of laminating the non-woven fabric and the PTFEporous film, there are a method of spray coating an adhesive onto thenon-woven fabric and laminating the non-woven fabric with the PTFEporous film and a method of laminating them by heating, but the methodis not limited to these methods.

[0026] Then, the present invention is practically explained by thefollowing examples.

[0027] In addition, the quantitative analysis of the generated gas (DBP,BHT, hydrocarbon, toluene, etc.) by gas chromatography (GC) in theexamples was carried out by introducing a gas generated using a purge &trap type Curie point headspacer sampler (JHS-100A, trade name,manufactured by Nippon Bunseki Kogyo K.K.) to a GC (GC-17A, trade name,manufactured by SHIMADZU CORPORATION) for analysis. The measurementconditions were as follows.

[0028] Headspace Sampler Conditions

[0029] Primary desorption temperature: 120° C.×20 minutes

[0030] Secondary adsorption temperature: −40° C.

[0031] Secondary desorption temperature: 358° C.×30 seconds

[0032] GC Measurement Conditions

[0033] Column: UA-1 0.25 mmφ×30 m, df=0.25 μm

[0034] Detector: FID

[0035] Column temperature: 45° C.→(3 minutes) 10° C./minute→260° C. (3minutes)

[0036] Carrier gas: Helium

EXAMPLE 1

[0037] The four edges of each of ELEVES T0153WDO (trade name,manufactured by UNITIKA LTD.) and TAPYRUS SN32ESB-SHP10 (trade name,manufactured by Tonen Tapyrus K.K.) were fixed by a metal frame, andthey were placed in an oven of an atmospheric temperature of 80° C. for20 minutes to perform a heat treatment.

[0038] Then, 100 parts by weight of a PTFE fine powder (Fluon CD-123,trade name, manufactured by Asahi•ICI Fluoropolymers K.K.) and 30 partsby weight of a liquid lubricant (liquid paraffin) were uniformly mixed.The mixture was pre-molded under the condition of 20 kg/cm², thenextrusion molded into a rod-form, and further the rod-form product waspassed between a pair of metal-made rolling rolls to obtain a longsheet-form molding having a thickness of 0.2 mm. Then, after removingthe liquid lubricant from the sheet-form molding by an extrusion methodusing Trichlene, the sheet-form molding was wound round a tubular corein a roll form.

[0039] The sheet-form molding was drawn 30 times in the lengthwisedirection at 370° C. by a roll stretching method. Then, the sheet-formPTFE molding was drawn 9 times to the width direction at 100° C. using atenter to obtain a PTFE porous film of a sintered state. The PTFE porousfilm was laminated with the heat-treated non-woven fabrics describedabove by a thermal lamination such that the PTFE porous film wasinterposed between the non-woven fabrics.

[0040] About the PTFE porous film air filer obtained as described above,the quantitative analysis of generated gases were carried out. Theresults are shown in Table 1 below. TABLE 1 Amounts of Generated Gases(μg/g) BHT DBP Total Example 1 — — 4

EXAMPLE 2

[0041] The PTFE porous film prepared in Example 1 was laminated withELEVES T0153WDO (manufactured by UNITIKA LTD.) and TAPYRUS SN32ESB-SHP10(manufactured by Tonen tapyrus K.K. ), which were not heat-treated, by athermal lamination such that the PTFE porous film was interposed betweenthe non-woven fabrics.

[0042] The four edges of the PTFE porous film air filter prepared asdescribed above were fixed by a metal flame, the air filter was placedin an oven of an atmospheric temperature of 120° C. for 5 minutes toperform a heat treatment and the quantitative analysis of the generatedgases was carried out. The results are shown in Table 2 below. TABLE 2Amounts of Generated Gases (μg/g) BHT DBP Total Example 2 — — 6

COMPARATIVE EXAMPLE 1

[0043] The PTFE porous film of the sintered state prepared in Example 1was laminated with ELEVES T0153WDO (manufactured by UNITIKA LTD.) andTAPYRUS SN32ESB-SHP10 (Tonen Tapyrus K.K.), which were not heat-treated,by a thermal lamination such that the PTFE porous film was interposedbetween the non-woven fabrics.

[0044] About the PTFE porous film air filter obtained as describedabove, the quantitative analysis of the generated gases was carried out.The results are shown in Table 3 below. TABLE 3 Amounts of GeneratedGases (μg/g) BHT DBP Total Comparative 3.5 0.15 29 Example 1

[0045] As is clear from the results shown in Tables 1 to 3, it can beseen that in Examples 1 and 2 wherein the heat treatment was carriedout, the PTFE porous film air filters giving less amounts of generatedgases were obtained as compared with Comparative Example 1 wherein aheat treatment was not carried out.

[0046] As described above, a PTFE porous film air filter having anexcellent property of not causing a problem of generating harmful gases,such as BHT and DBP, can be provided by laminating a non-woven fabricmade up of fibers containing at least one kind selected from polyolefinsand polyesters as the main components or a composite of these non-wovenfabrics with a PTFE porous film after heat-treating the non-woven fabricor the composite of the non-woven fabrics at a temperature of from 80°C. to 130° C. for from 5 minutes to 20 minutes or by effecting such aheat-treatment after the lamination according to the present invention.

[0047] While the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. An air filter comprising a laminate of a non-woven fabric made up of fibers containing at least one selected from polyolefins and polyesters as the main component or a composite composed of the non-woven fabrics and a polytetrafluoroethylene porous film, wherein the total amount of gases generated from the air filter is not more than 20 μg/g as a result of quantitative analysis by gas chromatography.
 2. An air filter of claim 1, wherein the non-woven fabric or the composite thereof is heat-treated.
 3. A production method of an air filter, which comprises heat-treating a non-woven fabric made up of fibers containing at least one kind selected from polyolefins and polyesters as the main component or a composite of the non-woven fabrics at a temperature of from 80° C. to 130° C. for from 5 minutes to 20 minutes and thereafter laminating the non-woven fabric or the composite thereof with a polytetrafluoroethylene porous film.
 4. A production method of an air filter, which comprises laminating a non-woven fabric made up of fibers containing at least one kind selected from polyolefins and polyesters as the main component or a composite of the non-woven fabrics and a polytetrafluoroethylene porous film, and thereafter heat-treating the resulting laminate at a temperature of from 80° C. to 130° C. for from 5 minutes to 20 minutes. 